In recent years, photovoltaic solar cells have been produced from ultra pure virgin electronic grade polysilicon (EG-Si) supplemented by suitable scraps, cuttings and rejects from the electronic chip industry. As a result of the recent downturn experienced by the electronics industry, idle polysilicon production capacity has been adapted to make available lower cost grades suitable for manufacturing PV solar cells. This has brought a temporary relief to an otherwise strained market for solar grade silicon feedstock (SoG-Si) qualities. With demand for electronic devices returning to normal levels, a major share of the polysilicon production capacity is expected to be allocated back to supply the electronics industry, leaving the PV industry short of supply. The lack of a dedicated, low cost source of SoG-Si and the resulting supply gap developing is today considered one of the most serious barriers to further growth of the PV industry.
In recent years, several attempts have been made to develop new sources for SoG-Si that are independent of the electronics industry value chain. Efforts encompass the introduction of new technology to the current polysilicon process routes to significantly reduce cost as well as the development of metallurgical refining processes purifying abundantly available metallurgical grade silicon (MG-Si) to the necessary degree of purity. None have so far succeeded in significantly reducing cost of production while providing a silicon feedstock purity expected to be required to match the performance of PV solar cells produced from conventional silicon feedstock qualities today.
When producing PV solar cells, a charge of SoG-Si feedstock is prepared, melted and directionally solidified into a square ingot in a specialised casting furnace. Before melting, the charge containing SoG-Si feedstock is doped with either boron or phosphorus to produce p-type or n-type ingots respectively. With few exceptions, commercial solar cells produced today are based on p-type silicon ingot material. The addition of the single dopant (eg. boron or phosphorus) is controlled to obtain a preferred electrical resistivity in the material, for example in the range between 0.5-1.5 ohm cm. This corresponds to an addition of 0.02-0.2 ppma of boron when a p-type ingot is desired and an intrinsic quality (practically pure silicon with negligible content of dopants) SoG-Si feedstock is used. The doping procedure assumes that the content of the other dopant (in this example case phosphorus) is negligible (P< 1/10 B).
If a single doped SoG-Si feedstock of a given resistivity is used in various addition levels the charge, the addition of dopant is adjusted to take into account the amount of dopant already contained in the pre-doped feedstock material.
Single doped feedstock qualities of n- and p-type can also be mixed in the charge to obtain a so-called “compensated” ingot. The type and resistivity of each component of the charge mix must be known to obtain desired ingot properties.
After casting, the solidified ingot is cut into blocks with the footprint of the resulting solar cells for example with a surface area of 125 mm×125 mm). The blocks are sliced into wafers deploying commercial multi-wire saw equipment.
PV solar cells are produced from the wafers in a number of process steps of which the most important are surface etching, POCl3 emitter diffusion, PECVD SiN deposition, edge isolation and the formation of front and back contacts.